A number of enzymatic processes have been identified in recent years which act to repair lesions in DNA induced by alkylating agents. These enzymes include the glycosylases, Apurinic/Apyrimidinic endonucleases and the methyltrans-ferases. The biological consequences of defects in these enzyme systems are for the most part unknown. Few mutants have been identified which are defective in these processes. We are currently using the bacteriophage Mud(Aplac) as a mutagene and screening for insertions of this phage which express the lac gene after treatment with sublethal levels of alkylating agents have accumulate strains containing such insertions. These can be either mutations in the enzymes responsible for repair of alkylated DNA as well as mutations in genes which regulate these enzymes or are required for their expression. Such mutants will permit the systematic evaluation of the relative roles these repair enzymes perform in the recovery from alkylation damage, evaluation of the mutagenic properties of the lesions upon which specific enzymes act and identification of the genes which code for these enzymes.